Bit-Stream Control of Doubly Fed Induction Generators
In recent years rising energy costs and concerns about global climate change have led to the widespread development of renewable electricity generation technology, especially wind generation. The Doubly Fed Induction Generator (DFIG) is commonly used for wind generation applications as it allows for efficient operation of the wind turbine by varying the speed of the generator without resorting to the use of large power converters. A typical DFIG system employs a wound rotor induction generator, with a stator that is directly connected to an external AC grid. The rotor winding is connected to the AC grid via a pair of back-to-back power converters: the Rotor Side Converter (RSC) and Grid Side Converter (GSC). The RSC and GSC require separate vector control systems. Traditionally, these control systems are implemented using microcontrollers, which perform the required control functions sequentially. As the complexity of control algorithms increases to achieve improved functionality and the time available to complete the calculations reduces to keep pace with ever-faster power electronic devices, microprocessor based solutions cannot execute the control program within the available time limit. A potential solution to this performance limitation is the use of hardware based control systems, which can execute control algorithms extremely quickly because many operations are processed in parallel. This is attractive for use in control systems, but implementing hardware control systems is difficult. The designer must either manually translate the control algorithm into individual operations, which are then usually encoded using a Hardware Description Language (HDL), or employ an automated code generation tool. The code is then synthesised and used to program a Field Programmable Gate Array (FPGA). An alternative method for implementing digital control systems is the Bit-Stream control technique. In contrast to the usual methods of implementing digital controllers, Bit-Stream control systems are developed using standard schematic editors provided by FPGA manufacturers. Provided that the desired control functions are available in the Bit-Stream library, the designer need not use HDL to develop the control system. A Bit-Stream based control system for a 2.6 kW DFIG is proposed in this thesis. As previously published research on Bit-Stream control systems focused on scalar control systems only, new Bit-Stream blocks are developed for use in vector control systems. The new vector control blocks include: coordinate transformation units, a Phase Locked Loop (PLL) and a family of modulators which convert Bit-Stream reference signals to gate drive commands for three-phase inverters. A Bit-Stream control system for a 2 kW GSC is developed, simulated and tested experimentally. Elements of this control system are adapted and re-used to develop a Bit-Stream based control system for the RSC of a 2.6 kW DFIG system. These are the first Bit-Stream based vector control systems to be reported in the literature. Supervisory control systems are added to the GSC and RSC to provide closed-loop speed control of the generator speed, soft synchronisation of the generator to the AC grid and operation with unity power factor at the stator terminals of the generator. Control of the DFIG is demonstrated by simulation and experimental testing of a 2.6 kW DFIG system.
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